Method for transmitting an additional data signal and a useful data signal in an optical network

ABSTRACT

A method for transmitting an additional data signal and a useful data signal, via optical connections, in an optical network, wherein the additional data signal is transmitted on the same channel as the useful data channel and converted into code sequences which are, in turn, superimposed on the useful data signal with a small amplitude. At the reception end, the additional data signal is recovered by cross-correlation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of transmitting both a usefuldata signal and an additional data signal on the same channel whereinthe additional data signal is first converted into code sequences whichare superimposed on the useful data signal with a small amplitude.

2. Description of the Prior Art

In communication networks with non-permanent connections, eachconnection is provided with an identifier in order to be able todetermine whether erroneous connections are occurring as a result offaults in the exchanges. This check is necessary particularly in thecase of semi-permanent connections, such as are produced bycross-connectors (network nodes), for example in the synchronous digitalhierarchy SDH. These identifiers also need to be transmitted in the caseof transparent fully-optical networks in which no access to additionalinformation transported together with the signal as part of thetransmission is possible, said information carrying the connectionidentifier, for example the source identifier, in the existing networks.

German Offenlegungsschrift DE 3 522 130 A discloses a method fortransmitting an additional channel in optical transmission systemswherein the additional channel is created without an additionalbandwidth of requirement by repeatedly recoding the useful data.However, an appropriate device is then required to recover the usefuldata.

A further solution is to transmit the identifier on a separateadditional channel which is routed parallel to the monitoring connectionin all network elements. European patent application EP 0 735 705 A1discloses a service channel transmitted using a different wavelength.This necessitates an additional frequency requirement. The disadvantageof this method is that it is not possible to prevent the additionalchannel, transmitting the identifier, and the useful signal from beingallocated separately or incorrectly on account of a fault in a networkelement; for example, of the cross-connector. This means, of course,that reliable detection of an erroneous connection is no longerpossible. It is possible, however, for an erroneous connection to besimulated.

The problem, therefore, is that reliable association of the useful datasignal and the additional data signal is necessary with opticalconnections. To do this, the transmission of useful data, described inEP 0 735 705, needs to be modified.

SUMMARY OF THE INVENTION

Accordingly, pursuant to the teachings of the present invention, boththe useful data and the additional data are transmitted on the samechannel wherein association is always guaranteed. Transmission on thesame channel is enabled as a result of the additional data signal firstbeing converted into code sequences which are then superimposed on theuseful data signal with a small amplitude.

The use of a code signal converted to a higher frequency band can beadvantageous because it means that extremely low-frequency spectralcomponents are avoided. Problems in the lower frequency range of fiberamplifiers are avoided with a carried additional signal.

In all cases, superimposing the additional data signal on the usefuldata signal only results in the signal-to-noise ratio being slightlyworsened; the additional data signal having the effect of slightlyincreasing the channel noise. Even if the signal-to-noise ratio isgenerally poor as a result of transmission, the additional data signalstill can be detected. If each bit of the additional data signal isallocated a plurality of successive code sequences, the detectability isimproved further.

It is advantageous if the additional data signal is a binary signal asthis has the largest signal-to-noise ratio wherein the two logic statescan be transmitted as inverted code sequences.

It is also prefer that superimposition takes place only during theon-states of the useful data signal, if this signal is transmitted byway of keyed modulation. This is easier to implement [lacuna] in termsof circuitry.

The method can, in principle, be used with all types of modulation; forexample, with phase modulation.

The method is particularly suitable for transmitting identifiers oraddresses in optical transmission systems. The additional data signalcan be checked without accessing the useful data signal, so that it isconstantly possible to check the correctness of a connection. Aparticular identifier, i.e. a particular 1,0 sequence, can be repeatedlytransmitted without interruption so as to minimize the detection time.However, transmission using time-division multiplexing with furtheradditional data is also possible. Furthermore, the method also can beused, of course to transmit any desired additional signals.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the Detailed Description of thePreferred Embodiments and the Drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 shows a basic circuit diagram for the transmission of anadditional data signal in accordance with the method of the presentinvention;

FIG. 2 shows a timing diagram with the code sequences used and theuseful data signal; and

FIG. 3 shows a variant of the arrangement shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic circuit diagram of FIG. 1 shows a transmission device SEconnected via an optical network 10 to a reception device EE. A datainput 1 is used to supply a useful data signal DS, which also mayinclude a plurality of digital data signals, to an adder 3. To a secondinput of the adder 3, a coded additional data signal CDS is supplied.The two logic states of the binary coded additional data signal CDS areallocated two code sequences CF and {overscore (CF)}, which aresuperimposed on the useful data signal DS with a substantially smalleramplitude (approx. 0.1 to 0.01 of the amplitude of the useful datasignal). The summed signal SU produced in this way is transmitted as anoptical network 10 after conversion in an electro-optical converter.

The code sequence CF is produced by a code-sequence generator 6. It iscomposed of a number of so-called chips CHIP (logic states) which eachhave a substantially longer period duration than the individual bits ofthe useful data signal (for example 10 to 100 times). A plurality ofthese chips form the code sequence CF (FIG. 2, a). In the case of binaryadditional data, logic 1 has this code sequence allocated to it, forexample, whereas logic 0 is represented by the inverted code sequence{overscore (CF)} (FIG. 2, b). As a rule, a plurality of successive codesequences are allocated to one bit of the additional data. Like the chiplength, this is dependent on the transmission conditions and therelative amplitude of the additional data signal. The code sequence CFis intended to have an autocorrelation function which is as pulsed aspossible in order to allow reliable detection in the receiver usingcross-correlation.

The chip clock at the frequency f_(CHIP) is produced by a firstfrequency generator 5 or derived from an existing clock. The bit clockfor the additional data signal is derived from the chip frequency by afirst frequency divider 7 and is supplied to an identifier generator 8.The identifier produced by the latter corresponds to an address, forexample, which determines the code sequences as an additional datasignal ZDS via an EXCLUSIVE-NOR gate 9 (unchanged for every logic 1 orinverted for every logic 0).

In addition, it should be noted that an external additional signal ZSalso can be transmitted instead of the identifier KE; the signal ZSbeing supplied to the EXCLUSIVE-NOR gate 9 instead of the identifier.This also can contain information about the type of useful data signalstransmitted; e.g., ATM. It is also possible to transmit the two signals,or a plurality of signals, using time-division multiplexing.

Instead of the identifier, it is also possible to transmit any desiredadditional data ZS which can be fed in via an additional data input 2.This also can be done using time-division multiplexing.

In the receiver EE, the received optical signal is converted into theoriginal (not taking into account faults on the transmission path)summed signal SU again in an opto-electrical converter 11. This signalcorresponds to a slightly noisy useful data signal DS and is output assuch at the data output 12 for further processing.

A low-pass filter 13 splits off the additional data signal, whose bitsinclude, of course, the code sequences CF and {overscore (CF)} from thesummed signal. A correlator 14 compares the code sequences with the codesequence produced by a second code-sequence generator 16, the lattercode sequence corresponding to the code sequence produced at thetransmission end and having the same chip frequency, which is producedby a second frequency generator 17. The signals output by the correlatorare evaluated, in terms of magnitude, by a rectifier 21 (or acorresponding circuit) and are supplied via a threshold value decisionunit 22 to a controller 23. The controller 23 ensures, using a delayelement 15 (or a corresponding controller for the code-sequencegenerator 16), that the code sequences and a second frequency divider18, which controls an additional-data/identifier detector 19, runsynchronously. In the steady state, the threshold value can correspondto the maximum or minimum correlation value.

Instead of the control loop, the correlator can be designed as a shiftregister whose stored information is compared with the code sequences CFand {overscore (CF)}. The comparison determines whether one of the twocode sequences has been received.

The additional-data/identifier detector 19 evaluates the informationoutput by the correlator and either converts it into the additionalinformation ZS or assesses a longer bit sequences in order to output anacknowledgement signal KEE at the output 20 when a particular identifieris received. In addition, it should be noted that the identifier alsocan be formed using different code sequences. Instead of individualbits, it is also possible to convert bit combinations into various codesequences; for example, four bit pairs 00, 01, 10 and 11 into four codesequences superimposed on the useful data signal.

FIG. 3 shows an arrangement which essentially corresponds to FIG. 1 butin which the code sequences CF, {overscore (CF)} produced by theadditional data signal are additionally converted, in a mixer 25 withthe aid of a subcarrier at the frequency f_(SUB), into a desiredfrequency band offering transmission advantages. The reception devicethen needs to have an appropriate filter device, the bandpass filter 26,and a detector 27 to demodulate the additional data signal. Using thismethod, it is possible to transmit different additional data signals indifferent additional frequency bands.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

We claim:
 1. A method for transmitting an additional data signal and auseful data signal in an optical network, the method comprising thesteps of: converting, at a transmission end, the additional data signalinto a coded additional data signal having a smaller amplitude than thatof the useful data signal and having defined code sequences, each of thedefined code sequences including a plurality of chips and having a lowerchip frequency than a bit frequency of the useful data signal;superimposing the coded additional data signal on the useful data signalto form a summed signal; transmitting the summed signal; separating thecoded additional data signal from the useful data signal at a receptionend; comparing the coded additional data signal with the defined codesequences; reconverting the coded additional data signal into theadditional data signal; and outputting the additional data signal as anadditional signal.
 2. A method for transmitting an additional datasignal and a useful data signal in an optical network as claimed inclaim 1, the method further comprising the steps of: evaluating theadditional data signal; and outputting the additional data signal as anactuation signal.
 3. A method for transmitting an additional data signaland a useful data signal in an optical network as claimed in claim 1,further comprising the steps of: converting, at the transmission end,the coded additional data signal to a higher frequency band; filteringthe coded additional data signal out, at the reception end, of thesummed signal; and reconverting the coded additional data signal to thebase band frequency by demodulation.
 4. A method for transmitting anadditional data signal and a useful data signal in an optical network asclaimed in claim 1, wherein the additional data signal transmitted is abinary signal having first and second logic states, the first logicstate being allocated a first code sequence including a plurality ofchips, and the second logic state being allocated a second code sequencewhich is inverse of the first code sequence.
 5. A method fortransmitting an additional data signal and a useful data signal in anoptical network as claimed in claim 1, wherein one bit of the additionaldata signal has a plurality of successive code sequences allocated toit.
 6. A method for transmitting an additional data signal and a usefuldata signal in an optical network as claimed in claim 1, wherein theuseful data signal is transmitted via on/off keying, and wherein theadditional data signal is superimposed on the useful data signal with asmaller amplitude.
 7. A method for transmitting an additional datasignal and a useful data signal in an optical network as claimed inclaim 6, wherein the superimposition occurs only when the useful datasignal is keyed on.
 8. A method for transmitting an additional datasignal and a useful data signal in an optical network as claimed inclaim 1, wherein the additional data signal transmitted is an identifierwhich includes a plurality of bits.
 9. A method for transmitting anadditional data signal and a useful data signal in an optical network asclaimed in claim 8, wherein different code sequences are transmitted fordifferent identifiers.
 10. A method for transmitting an additional datasignal and a useful data signal in an optical network as claimed inclaim 8, wherein the identifier includes information about at least oneof a source of the useful data signal, a sink of the useful data signal,and a type of the useful data signal.
 11. A method for transmitting anadditional data signal and a useful data signal in an optical network asclaimed in claim 1, wherein the useful data signal transmitted is anexternal additional signal.
 12. A method for transmitting an additionaldata signal and a useful data signal in an optical network as claimed inclaim 1, wherein the additional data signal transmitted is an identifierwhich includes a plurality of bits and information about at least one ofa source of the useful data signal, a sink of the useful data signal,and a type of the useful data signal, wherein the useful data signaltransmitted is an external additional signal, and wherein both theidentifier and the external additional signal are transmitted usingtime-division multiplexing.
 13. A method for transmitting an additionaldata signal and a useful data signal in an optical network as claimed inclaim 1, further comprising the step of: frequency modulating at leastone additional data signal; and combining the at least one frequencymodulated additional data signal with the useful data signal.